1. Field of the Invention
The present invention relates to an improvement in a structure of a vibration wave motor driven by a travelling vibration wave.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,019,073, a vibration wave motor transduces a vibration motion generated in an electrostrictive element when a periodic voltage is applied thereto to a rotational motion or a linear motion. Since, unlike a conventional electric motor, a vibration wave motor does not require a winding it is simple in construction and compact in size and produces a high torque even at a low rotating speed and has a low inertia moment.
However, in the known vibration wave motor, in transducing the vibrative motion to rotational motion, a movable member such as a rotor which contacts a vibration member is unidirectionally friction-driven by a standing vibration wave generated in the vibration member. In a forward movement of the vibration member, the movable member frictionally contacts the vibration member, and in a backward or return movement, the movable member is moved away from the vibration member. Thus, the vibration member and the movable member must be constructed such that they contact each other within a very small distance range, that is in a point contact or a line contact. As a result, the efficiency of the friction drive is very low.
Further, since the drive force acts only in a given direction, a direction of movement of the movable member is unidirectional. In order to move the movable member reversely, it is necessary to mechanically switch the direction of vibration by another vibration member. Thus, in order to attain a reversibly rotatable vibration wave motor, a complex apparatus is required and the advantages of the vibration wave motor, that is, simple construction and compactness are substantially lost.
In order to resolve the above problem, a vibration wave motor driven by a travelling vibration wave has recently been proposed.
FIG. 1 shows a developed view of such a vibration wave motor.
A vibration absorber 4, a metal ring vibration member 2 having electrostrictive elements 3 arranged thereon and a vibration member 1 are inserted, in this sequence, to a central cylinder 5a of a stator 5 serving as a base. The stator 5, the absorber 4 and the vibration member 2 are mounted such that they do not rotate relative to each other. The movable member 1 is pressed to the vibration member 2 by its gravity or biasing means, not shown, to maintain the integrity of the motor. A plurality of electrostrictive elements 3a are arranged at a pitch equal to one half of a wavelength .lambda. of a vibration wave, and a plurality of electrostrictive elements 3b are also arranged at a pitch of .lambda./2. The plurality of electrostrictive elements 3 may be a single ring-shaped element polarized at the pitch of .lambda./2 to form polarized areas 3a and 3b as shown in FIG. 2. The electrostrictive elements 3a and 3b are phase-differentially arranged at a pitch of (n.sub.o +1/2).lambda., where n.sub.o =0, 1, 2, 3, . . . Lead wires 11a are connected to the respective electrostrictive elements 3a and lead wires 11b are connected to the respective electrostrictive elements 3b, and the lead wires 11a and 11b are connected to an AC power supply 6a and a 90.degree. phase shifter 6b (see FIG. 3). A lead wire 11c is connected to the metal vibration member 2 and it is connected to the AC power supply 6a. A friction area 1a of the movable member 1 is press-contacted to the vibration member 2 formed with a hard surface to enhance a frictional force and reduce abrasion.
FIG. 3 illustrates the generation of the vibration wave in the motor of FIGS. 1 and 2. While the electrostrictive elements 3a and 3b bonded to the metal vibration member 2 are shown adjacent to each other for the sake of convenience of explanation, they meet the requirement of phase difference of .lambda./4 described above and are essentially equivalent to the arrangement of the electrostrictive elements 3a and 3b of the motor shown in FIG. 1. Symbols .sym. in the electrostrictive elements 3a and 3b indicate that they expand in a positive cycle of the AC voltage and symbols .crclbar. indicate that they shrink in the positive cycle.
The metal vibration member 2 is used as one of electrodes for the electrostrictive elements 3a and 3b, an AC voltage of V=V.sub.0 sin wt is applied from the AC power supply 6a to the electrostrictive elements 3a, and an AC voltage of V=V.sub.0 sin (.omega.t.+-..pi./2) which is phase-shifted by .lambda./4 is applied to the electrostrictive elements 3b from the AC power supply 6a through the 90.degree. phase shifter 6b. A sign + or - in the equation is selected by the phase shifter 6b depending on the direction of movement of the movable member 1 (not shown in FIG. 3). When the sign + is selected, the phase is shifted by +90.degree. and the movable member 1 is moved forwardly, and when the sign - is selected, the phase is shifted by -90.degree. and the movable member 1 is moved reversely. Let us assume that the sign - is selected and a voltage of V=V.sub.0 sin (.omega.t-.pi./2) is applied to the electrostrictive elements 3b. When only the electrostrictive elements 3a are vibrated by the voltage of V=V.sub.0 sin .omega.t, a standing vibration wave is generated as shown in FIG. 3(a), and when only the electrostrictive elements 3b are vibrated by the voltage of V=V.sub.0 sin (.omega.t-.pi./2), a standing vibration wave as shown in FIG. 2(b) is generated. When the two AC voltages having the phase difference therebetween are simultaneously applied to the electrostrictive elements 3a and 3b, the vibration wave travels. FIG. 3(c) shows a wave at time t=2n.pi./.omega., FIG. 3(d) shows a wave at t=.pi./.omega.+2n.pi./.omega. and FIG. 3(f) shows a wave at t=3.pi./2.omega.+2n.pi./.omega.. A wavefront of the vibration wave travels in an x-direction.
The travelling vibration wave includes a longitudinal wave and a lateral wave. At a mass point A of the vibration member 2 shown in FIG. 4, a longitudinal amplitude u and a lateral amplitude w causes a counterclockwise rotating elliptic motion. Since the movable member 1 is press-contacted to the surface of the vibration member 2 and it contacts to only an apex of the vibration plane, it is driven by components of the longitudinal amplitude u of the elliptic motion of the mass points A, A', . . . at the apexes so that the movable member 1 is moved in a direction of an arrow N.
When the phase is shifted by 90.degree. by the 90.degree. phase shifter, the vibration wave travels in a -x direction and the movable member 1 is moved oppositely to the arrow N.
In this manner, in the vibration wave motor driven by the travelling vibration wave, the forward and reverse directions of rotation can be switched with a very simple construction.
A velocity of the mass point A at the apex is represented by V=2.pi.fu (where f is the vibration frequency). A velocity of the movable member 1 depends on the velocity of the mass point A and also on the lateral amplitude w because of the frictional drive by the press-contact. Thus, the velocity of the movable member 1 is proportional to the magnitude of the elliptic motion of the mass point A, and the magnitude of the elliptic motion is proportional to the voltage applied to the electrostrictive elements.
However, as described above, in the prior art vibration wave motor, a resonance frequency in an inner circumference of the vibration member 2 is different from that in outer circumference because the thickness of the vibration member 2 is constant. Accordingly, in the prior art vibration wave motor, the resonance frequency of the vibration member 2 is that for a circumference of a certain radius between the inner circumference and the outer circumference and it impedes the resonance at a circumference other than the certain radius.
The resonance frequency f is further discussed. ##EQU1## where f (=2.pi..omega.) is a frequency of the input voltage, E is a Young modulus of the vibration member 2, .rho. is a density and h is a thickness. Thus, the resonance occurs at the thickness h which meets the above relation. Because the vibration member 2 is of ring shape, the equation (1) is satisfied within a small ring width range in a diameter D of the ring, and the circumferential length .pi.D resonates at n.lambda., where .lambda. is a wavelength and n is a natural member. ##EQU2## Accordingly, from the equations (1) and (2), ##EQU3## Since the thickness of the vibration member 2 is constant in the prior art vibration wave motor, only the diameter which meets the following modification of the equation (3) resonates. ##EQU4##